A mass spectrometer generally includes an inlet slot following which the beam passes through an electrostatic sector, and then a magnetic sector. The aim of this disposition is to deflect the particles in a manner which separates them as much as possible as a function of their masses, and which is as insensitive as possible to their kinetic energies. The deflection takes place in a "radial" plane which is the plane of symmetry of the instrument and which is perpendicular to the long direction of the inlet slot. The particle beam thus has a radial component and a perpendicular component in the so-called vertical section.
It is known to use a magnetic sector which possess an inlet face and an outlet face which are both planar, and in which the inlet face is inclined to the axis of the particle beam, while the plane of the outlet face passes through the intersection of the inlet face and the particle beam. Under these conditions, the deflection angle to the particles passing through the magnetic sector does not depend on the mass of the charged particles, thereby simplifying the system. The radius of curvature of the trajectories, however, does depend on the mass of the particles.
The quality of a mass spectrometer is defined by its separating power M/.DELTA.M, where .DELTA.M is the smallest mass difference which can be distinguished by the instrument. In a spectrometer having perfect optics (the term "optics" is used in a broad sense here), this separating power would only depend on the size of the inlet slot. In reality, the images of the inlet slot, or "rays", are deformed by optical defects in the apparatus known as aberrations. These aberrations depend mainly on the energy dispersion .DELTA.V of the ions, and on the aperture of the beam which is limited by the inlet slot, generally, the inlet plot is inserted prior to the magnetic sector.
For a given separating power, the best spectrometer is the most sensitive, i.e. the spectrometer which accepts the beam whose geometrical extent is the greatest. This aptitude of the spectrometer is called "clarity". However, for a given spectrometer geometry, the clarity can only be increased by reducing the undesirable effects of aberrations.
Finally, when it is desired to perform simultaneous measurements on all the rays of the spectrum (all the masses), the correction or elimination of aberrations is much more difficult.
The problem is thus one of providing a high clarity mass spectrometer capable of simultaneous multiple detection and which possesses a high separating power.
To this end, a first aim of the present invention is to correct the aberrations of the spectrometer, in particular of its magnetic sector, but also of its electrostatic sector.
A second aim of the present invention is to improve the matching and the transfer of the ion beam to the input of the spectrometer by means of transfer optics placed upstream from the mass spectrometer per se.
These aims are achieved by various aspects of the invention, some of which are of interest on their own account.
In known manner, the apparatus proposed herein comprises an inlet slot, followed by an electrostatic sector, and then a magnetic sector. An aperture slot may be inserted between the electrostatic and the magnetic sectors, or else, in conventional manner, at the inlet to the electrostatic sector. This assembly serves to deflect a particle beam in a radial plane perpendicular to the long dimension of the inlet slot. The magnetic sector has a planar inlet face which is inclined to the axis of the particle beam and an outlet face which is also plane and whose planar passes through the intersection of the inlet face and the particle beam. The planes in question are the effective magnetic faces which differ from the material faces because of the leakage fields.
Further, it is also known in a particular context, to interpose a quadrupole between the electrostatic and the magnetic sectors (see H. MATSUDA, MASS SPECTROMETRY REVIEWS, Vol. 2, No. 2 (1983) John Wiley, pages 289 to 325). However, this quadrupole operates very differently from that used by the present invention.
Finally, mass spectrographs of the "Mattauch-Herzog" type are also known, in which the aperture and the width of the energy band are respectively delimited by means of an aperture diaphragm and an "energy diaphragm" whose adjustments interact. The ranges over which the aperture may vary and over which the energy bandwidth may vary must be reduced to relatively small values if second order aberrations are to remain negligible. As a result, the transmission or clarity of the apparatus is small.
In French patent application published under No. 2 056 163, there is a proposal to transform a mass spectrograph of the Mattauch-Herzog type in such a manner as to enable independent adjustment of the aperture and of the energy bandwidth, and at the same time obtaining a greater value for the clarity of the apparatus.
The means proposed by this prior French document consists in disposing a first electric lens (18) between the inlet slot (10) and the energy diaphragm (20), and in disposing a second electric lens (22) between the energy diaphragm (20) and the magnetic sector (24). The specification gives details of the role of these lenses relative to the aperture diaphragm and to the adjustable energy diaphragm.
In one of the embodiments described by this specification the inlet face of the magnetic sector (24) is inclined at an angle .epsilon. which is equal to 26.6.degree.. This use of an inclined inlet face, which is in itself well known, serves to define a focal point which is at a distance of twice the radius in a direction perpendicular to the plane of symmetry of the apparatus. The focus plane (26) is shifted behind the magnetic field through an angle w which is equal to 8.1.degree. in this case.
The value of 26.6.degree. for the angle .epsilon. corresponds to a standard deflection of 90.degree. by the magnet. Naturally, a different value for the deflection angle would give rise to a different value for the angle .epsilon. in this prior document. This value of 26.6.degree. is close to that described below in relation to the present invention, but for other reasons and using a magnetic sector receiving a particle beam which is parallel to the radial plane.
However, prior French patent application No. 2 056 163 uses "other way" focusing: whereas in a normal mass spectrograph, the returning particles are lost, in this prior specification the lenses return them by preventing the beam from moving away. It thus appears that the inclination of the inlet face of the spectrograph is used in the prior document to tighten the beam and not to correct aberrations therein as is proposed by the present invention.